First of all, kudos to you for being so honest. And I think the other answers are great advice. However, I think the following points are yet missing. You ask:

When I meet someone to date, how can I communicate clearly [...]

The key point here is that this communication should not be a one-time thing you tell potential candidates only on your first date. There is no magic sentence that wards off emotions by your acquaintances for weeks, months or years. Especially if you send mixed signals:

"Relationship", "dating", etc.

The power of words should not be overestimated. Telling someone that you only want to 'have fun', while being there for them in difficult times, having sex together, sleeping in the same bed [I just assume you do this], being monogamous, among other things, sends signals that are very different from whatever you say. From my experience, people automatically expect romantic relationships to become more serious over time. This is not something you can change. This article even claims women automatically fall in love from sex alone.

From this I realized I didn't enjoy serious relationships, so I've avoided them.

Avoidance behaviour is rarely effective. This way, you are dealing with emotions, but not solving the problem. You're a firefighter spraying yourself (in order to cool yourself) instead of extinguishing the fire. You are now approaching 30. Around this age, two problems emerge for what I assume to be your current lifestyle:

1. Your friends develop serious relationships and/or mature.
2. Women expect 'more' from men of your age.

To problem 1: Depending on your social circle, this might come sooner, later or might have already happened: Couples move in together, marry, have kids. Singles stay at home, use dating apps, focus on their career or simply go to bed early because they have to work the next day. Point being: Your friends won't do as much cool stuff as they used to. And they will prioritize their partners. It will be hard for you to have as much fun with your friends as you used to. Their serious partners offer a lot of possibilities to simply 'have a good time'. You deny yourself these possibilites. Problem 2: As you have mentioned yourself, the women you are attracted to generally want a more serious relationship. See problem 1.

Solutions

The solution you want: Keep sex and friendship seperated, even if it means putting in more effort to continually find short-term sex partners and not being able to kiss your (female) friends at a concert.

The solution I consider more healthy: Seek psychotherapy. You already know what shapes your current relationship wishes: Your past relationships. That knowledge is a great starting point. However, you should ask yourself whether your future should be shaped by two people you don't even like anymore. A lot of people live great, satisfying relationships, free of cheating. It is possible for you, too. Also, you don't wake up with 35 and then you're magically able to open up to others emotionally and ready to sustain a serious relationship. You will have to face these problems sometime anyway.

First of all, kudos to you for being so honest. And I think the other answers are great advice. However, I think the following points are yet missing. You ask:

When I meet someone to date, how can I communicate clearly [...]

The key point here is that this communication should not be a one-time thing you tell potential candidates only on your first date. There is no magic sentence that wards off emotions by your acquaintances for weeks, months or years. Especially if you send mixed signals:

"Relationship", "dating", etc.

The power of words should not be overestimated. Telling someone that you only want to 'have fun', while being there for them in difficult times, having sex together, sleeping in the same bed [I just assume you do this], being monogamous, among other things, sends signals that are very different from whatever you say. From my experience, people automatically expect romantic relationships to become more serious over time. This is not something you can change. This article even claims women automatically fall in love from sex alone.

From this I realized I didn't enjoy serious relationships, so I've avoided them.

Avoidance behaviour is rarely effective. This way, you are dealing with emotions, but not solving the problem. You're a firefighter spraying yourself (in order to cool yourself) instead of extinguishing the fire. You are now approaching 30. Around this age, two problems emerge for what I assume to be your current lifestyle:

1. Your friends develop serious relationships and/or mature.
2. Women expect 'more' from men of your age.

Problem 1:
Depending on your social circle, this might come sooner, later or might have already happened: Couples move in together, marry, have kids. Singles stay at home, use dating apps, focus on their career or simply go to bed early because they have to work the next day. Point being: Your friends won't do as much cool stuff as they used to. And they will prioritize their partners. It will be hard for you to have as much fun with your friends as you used to. Their serious partners offer a lot of possibilities to simply 'have a good time'. You deny yourself these possibilities.

Problem 2:
As you have mentioned yourself, the women you are attracted to generally want a more serious relationship. See problem 1.

Solutions

The solution you want: Keep sex and friendship separated, even if it means putting in more effort to continually find short-term sex partners and not being able to kiss your (female) friends at a concert.

The solution I consider more healthy: Seek psychotherapy. You already know what shapes your current relationship wishes: Your past relationships. That knowledge is a great starting point. However, you should ask yourself whether your future should be shaped by two people you don't even like anymore. A lot of people live great, satisfying relationships, free of cheating. It is possible for you, too. Also, you don't wake up with 35 and then you're magically able to open up to others emotionally and ready to sustain a serious relationship. You will have to face these problems sometime anyway.